Graphics card or display?

I have an ATI X1600Pro (I know, I know) connected to a cheapo LCD monitor that I have had for about 2 years now. The problem is that I am now getting a 1.5 inch black bar all the way down the middle of the screen, that occasionally flashes, during the first portion of startup (mobo screen, pci device check, etc.) and during gaming. Desktop is fine, and nothing happens when doing more 'everyday' windows stuff (Word, Media Player, etc.). I'm wondering if this is the graphics card or the monitor going out. I tried switching from the digital monitor connection to the VGA, and I got the same thing -- well, the bar stayed solid-black more, with less flashing. The problem is hard to diagnose, because I only have the one monitor. I've rolled back Catalyst drivers, and I have also updated to the newest, but I get the same result. Any advice? Thanks!!!! (BTW, I have overclocked the card, on occasion, using atitool)
System Specs:
E4300@2.4 + Gigabyte DS3 mobo (temps are fine)
3GB Corsair XMS2Pro (running fine)
ATI HDTV Wonder (currently not working; maybe part of the prob?)
ATI X1600Pro 512 (culprit?)
Antec True Power Trio 550W (working fine)
USB and Firewire cards fill the other PCI slots (working fine)
2 HDDs (working fine)

If desktop is fine, but happens when gaming, then I would argue that it is most likely the card. If it was the monitor, then it would happen all the time, not just sometime. It seems to happen when the card is in 3D mode, or when the clocks are higher. To test this, try using an overclocking tool to down clock the card. The frame rates will be horrible, but if the bar goes away, you know what it is.

Its possible also that this is a heat problem. Open the case and remove the card. Blow out as much as the dust as you can. If it goes away then, you had a heat issue.

Try ... Playing with the resolutions ... Force the little bastard with 3DMark. Also check it up on another monitor ... I can't say more /few years ago I had similar problem with a CRT monitor ... / Good luck!

Thanks for the replies! Yeah Mactronix, that makes sense. I think I have fixed it, but I'm still not sure if I recognize the cause corectly. I have a dual-booter (XP pro and Vista pro), and the screen refresh rates were different under each OS. One was at 60hertz hertz, and the other was at 70hertz. I set them both to 70 hertz, and everything has been fine. This is strange, because the OS is not operating upon initial start-up, but maybe you all know something that I don't. Do the settings remain? Weird. My cooling is good, and temps are fair-to-good. I'm going to check on the HDTV Wonder this weekend and see what's going on. I rarely use it, so it has not bothered me much. Thanks again for all your help!!